ADuM5401 and Tri-State Input

I am using the CN-0336-EVAL board which has an AD7091R adc and a ADuM5401.  I'm having trouble with the MISO/SDATA being pulled low when the device is not selected.  It should go to high impedance after clocking the data out.  I was wondering, since the SDATA line goes to Vid on the ADuM5401 which outputs on Vod to my microcontroller, what does the ADuM5401 do with a high impedance input.  Does it output low on the Vod pin when the Vid pin is high impedance?  I have more than one device hooked to the SPI bus and it would be nice if the MISO/SDATA would get released.

Thanks for your help.

  • 0
    •  Analog Employees 
    on Sep 5, 2016 9:32 PM

    AFrank:

    The ADuM540x input channel needs to be set high or low to determine the correct output channel state.  In the case where the driver for the VID input is high impedance, it is advised that a pullup resistor be connected to VID, to set the input high, and the output VOD will follow the input and be high also. In the datasheet for the AD7091R figure 22  typical connection diagram, a 100kohm pullup is used.

    Regards, Brian

  • Thank you for your response and answer.  

    Please excuse my ignorance, I'm a firmware engineer so this is out of my depth. 

    Does this mean the VOD will not be high impedance when the AD7091R sdata like is high impedance?  

    Observing the VOD line on a scope other SPI devices are not able to pull the sdata line to full high, as if the VOD is driving the line low.  This is on the Analog Devices CN0336 EVAL board with additional SPI devices connected. 

    Thanks again for your answers and help. 

    Alanna Frank

    Sent from the Samsung Galaxy Rugby Pro, an AT&T LTE smartphone

  • 0
    •  Analog Employees 
    on Sep 6, 2016 10:14 PM

    Alanna Frank:

    The VOD will not be high impedance, it will be high to follow the state on the input VID, which in this case would have a pull-up resistor added, as we discussed.

    Regards, Brian

  • Okay, thank you. I appreciate it.

     

    Since the output is on the SPI bus and has to go tri-state so other devices can control the line also, I will put the eval board on a separate SPI bus, thank goodness I have a few spares.  I was hoping to use the Eval board out of the box without modifications, since I'm not electrical and suck at soldering.  And, I'm not sure if putting the pull up on the input would achieve what I need, since the other devices need the line also.

     

    Thanks again for your help.

    Alanna

  • Hi,

    Is it possible to share the SPI bus in CN0336 EVAL with other IC ?

    assuming the 100K pull-up resistor is present as suggested in figure 22, (although VOD will not be high impedance).